The present invention relates to an improvement in the process for preparing flame retardant polyamide molding resins that contain melamine cyanurate as a flame retarding agent which results in products with improved mechanical properties and good flame retarding properties. More particularly, it relates to an improvement which comprises the addition of a glycol and a phthalate ester to the process wherein a polyamide, melamine or derivatives thereof and cyanuric acid are melt-blended in the presence of water.
Polyamide molding resins are used in a wide range of applications because of their excellent mechanical strength. For electrical and other applications, a high level of flame retardancy is required along with excellent mechanical strength. For tie-strap and other related applications an even higher degree of mechanical strength, in particular, a higher degree of tensile elongation, is required.
Melamine and other triazines, and certain salts of said compounds such as melamine cyanurate, are known to function as flame retardants when melt-blended in polyamides at from about 5% by weight to about 20% by weight. It is believed that the manner by which the flame retardancy is brought about is through the thermal decoxposition of the additive (gas phase) which blankets the burn site so as to exclude oxygen.
Melamine cyanurate salt can be added directly to a molten polyamide resin or it can be formed during melt-blending of polyamide resins With
melamine or derivatives thereof and cyanuric acid in the presence of water. See U.S. Pat. No. 4,363,890.
The advantages of utilizing the latter process are the lower cost of melamine and cyanuric acid as compared to melamine cyanurate salt and the ability to form melamine cyanurate salt in situ having a smaller particle size than the commercially available product thereby increasing its dispersion in the polyamide and advantageously affecting the mechanical and flame retarding properties of the resulting resin.
The disadvantages are that when processing polyamides with higher melt temperatures it becomes very difficult to assure consistent melamine cyanurate salt formation. This result can be attributed to the water being volatilized during extrusion before the salt has a chance to form which necessitates the use of an excess of water or a closed melt-blending system. The excess water is undesirable and the closed system gives rise to other problems.
In addition, the mechanical strength of the resins produced by the latter process is inadequate for certain applications.